1. Field of the Invention
The present invention relates to a technology for increasing a breakdown voltage and reliability of a semiconductor power device, and more particularly to a packaging technology for preventing a dielectric breakdown of a power device module which has a plural of power switching devices mounted thereon and for increasing a breakdown voltage and reliability thereof.
2. Description of the Related Art
Semiconductor power devices for converting and controlling the flow of electric energy have been greatly researched and developed in recent years in response to high requirements from various fields including the power electronics field at first. It is particularly required to realize a power switching device, for example, a Power MOSFET, a Power Bipolar Transistor (Power BJT), a Gate Turn-off (GTO) Thyristor and an Insulated Gate Bipolar Transistor (IGBT) that can control a high power and achieve a high performance. As the result, a high voltage semiconductor power device called an Intelligent Power Module (IPM) is proposed and developed, which integrates a plural of power switching devices and a semiconductor chip that includes a circuit for controlling these devices in a single package. The IPM is currently applied to an inverter such as a direct drive circuit of an energy reduction type and an intelligent actuator, and is also expected to apply to other fields. In addition, it is particularly required to make the IPM have a much higher breakdown voltage.
To increase the breakdown voltage of such the semiconductor power device, it can be considered currently to improve a packaging technology that employs a substrate with a direct bond structure capable of achieving an advanced planning for heat radiation. The packaging technology that employs the substrate with the direct bond structure means such a packaging technology that employs a direct bond copper (DBC) substrate, which consists of a high heat conductive aluminum nitride (hereinafter, referred to as "an AIN") substrate and a copper film attached on the surfaces of the AIN substrate, and that can reduce a heat resistance and simplify the structure.
FIG. 1 is a cross sectional view showing a configuration of a package for a semiconductor power device with the DBC substrate. In the package, a plural of semiconductor chips 21, 22 and 23 are disposed on a copper film 1a that is attached on a top surface of an insulating AIN substrate 2 and another copper film 1b that is attached at a bottom surface of an insulating AlN substrate 2 except a peripheral region thereof. The semiconductor chips 21, 22 and 23 are connected electrically with each other via lead wires 31 and 32 such as bonding wires and ribbon wires. These semiconductor chips 21, 22 and 23 may, for example, include semiconductor power chips 21, 23 and a control circuitry chip 22 for controlling the semiconductor power chips 21 and 23.
The copper film 1b bonded to the bottom surface of the AIN substrate 2 is attached to the center of a metallic heat sink 5. A case (body of a container) 6 is equipped on a peripheral region of the heat sink 5 to surround the entire AIN substrate 2 as shown in FIG. 1.
A terminal holder 8 has external terminals (electrode terminals for external connection) 71, 72 and an aperture 8a, and is fixed at an upper portion inside the case (container body) 6 in order to close the outer case 6. The external terminals 71 and 72 are employed such members as to realize electrical conduction to external from each of the semiconductor power chips 21, 23 and the control circuitry chip 22 inside the case (container body) 6.
A silicone gel 91 or an insulating material is flowed through the aperture 8a into a module which is surrounded by the conductive bottom plate 5 used as the heat sink, the case (container body) 6 and the terminal holder 8. An epoxy resin is thereafter provided on the silicone gel to seal. After sealing with the epoxy resin, arranging the terminal holder 8 to close the case, which may consist of a material as same as or different from that of the outer case, then closing the aperture 8a with sealing materials 81 and 82 after setting the silicone gel 91.
In the particular case where the IPM is used in a high voltage power equipment, a hygroscopic property presented in the silicone gel 91 may degrade electric characteristics. Therefore, suitable adhering methods and adhering structures, which can suppress generation of openings and joints among the components by strong bonds between the outer case 6 and the heat sink (the conductive bottom plate) and between the lid (terminal holder) 8 and the epoxy resin, for example, are employed to prevent humidity from penetrating into the IPM as far as possible. An inner structure such as a space and a pressure-releasing valve is also provided inside the IPM to prevent the silicone gel 91 from swelling and exuding, and to reduce the heat stress applied to the epoxy resin.
The package structure shown in FIG. 1 may improve, by employing a high heat conductive AIN substrate, a thermal conductivity between the conductive bottom plate serving as the heat sink and the substrate, and may also reduce the amount of a molybdenum (Mo) plate and a solder material by bonding the semiconductor chip directly to the AlN substrate 2 that has the top copper film 1a. This is an advanced technology in the radiation planning which can reduce the heat resistance and simplify the structure as mentioned above.
However, the above-described conventional IPM has several technical problems that are derived from the structure thereof. These technical problems will be described below.
First, in order for undergoing to be used under a high voltage, the IPM adopts the adhering method or the adhering structure that can suppress the generation of the openings and joints between different components by the strong bonds. This configuration may play the role of reducing the moisture penetration. However, it is impossible to remove the generation of the openings and joints completely, leaving a tiny opening or joint within the semiconductor device. In addition, bonding the components strongly may increase the heat stress in a portion where different thermal expansion coefficients exist between components such as the outer case 6 and the heat sink (conductive bottom plate) 5, and may cause a breakage of the outer case 6 and an enlargement of the opening remained. The breakage of the outer case 6 and the enlargement of the opening may invite a moisture absorption by the silicone gel, resulting in a cause to degrade electrical properties and durability thereof.
Second, adding a component such as the pressure-releasing valve for relieving the heat stress inside the IPM may complicate an inner structure of the device and increase a size of the entire device.
Third, since the conventional IPM is produced from several materials such as the outer case 6, the lid (terminal holder) 8, the silicone gel 91 and the epoxy resin, the heat stress may be generated due to the differences among the thermal expansion coefficients of the materials, and a complicated force is applied to each portion inside the device, resulting in the breakage and the degradation of the substrate and electronic components in the device. In general, the steps that are required for assembling the device increases in accordance with the number of the materials used, causing a reduction of reliability of the product and an increase of cost.
Fourth, whereas the package structure of the conventional IPM has the advantage for planning the heat radiation to realize a high breakdown voltage, it leaves a problem with respect to the dielectric breakdown. Namely, the package structure mentioned above employs the silicone gel 91 as the insulating material, but the silicone gel 91 has a property in which a dielectric breakdown voltage is lower than those of general solid insulators. Because a creeping distance between the silicone gel 91 and the AlN substrate (a distance from an edge of the semiconductor power chip 21 or 23 to an edge of the AlN substrate 2) is short, a creeping breakdown at the interface between the both may occur. The adhesion at the interface between the both is insufficient, and a creeping discharge may occur easily.
Fifth, in the conventional semiconductor module, when the insulating substrate that receives the semiconductor chip is located directly on the bottom plate 5 of the metallic case (container body) that serves as the heat sink, the dielectric breakdown at the interface between the insulating substrate and the silicone gel may occur easily, because the creeping distance is short and the metallic bottom plate serves as a back electrode during the creeping discharge, extending the creeping discharge with ease.
Such the creeping breakdown and the creeping discharge do not occur when driven under a rated voltage, but are considered to occur during driven under a higher voltage than the rated voltage. Once the creeping breakdown or discharge occurs, the dielectric breakdown voltage is lowered and the dielectric breakdown may be easily caused.
In view of increasing the breakdown voltage and improving the reliability, therefore, suppressing the creeping breakdown and discharge to prevent the dielectric breakdown is required.